1. Field of the Invention
The present invention relates to a traction control system for a four-wheel steerable vehicle, comprising a reference value generating means for generating a reference value for detecting a slippage of a drive wheel in a driving state, and an output torque control means for controlling the torque of the drive wheel on the basis of an output from the reference value generating means so as to bring the drive wheel into a predetermined slipping state.
2. Description of the Prior Art
There is a traction control system conventionally known, for example, from Japanese Patent Publication Kokoku No. 48334/76, in which the slippage of the drive wheel in a driving state is detected on the basis of a reference value, and the torque of the drive wheel is reduced in accordance with such slippage, thereby preventing an excessive slipping of the drive wheel.
When the above traction control system is applied to a four-wheel steerable vehicle of a front wheel drive type, a good accelerating property similar to that in the prior art is achieved because the stability of the vehicle is excellent during driving with the front and rear wheels being in the same phase condition. However, when the front and rear wheels are turned from the same phase condition to a reverse phase condition or are held in the reverse phase, the understeering tendency of the vehicle is weakened and for this reason, it is necessary to reduce the lateral force of the front wheel to eliminate such tendency. It is known that the critical lateral force of a tire is reduced following an increase in slip rate, as shown in FIG. 5, when the slippage of the drive wheel is controlled. If the slip rate is controlled to a constant level, the critical lateral force becomes substantially constant. Therefore, when the front and rear wheels are turned from the same phase condition to the reverse phase condition or are held in the reverse phase, the lateral force of the front wheel becomes too large, thereby making it difficult to insure the steering stability.